The photovoltaic system is expected to become a “zero emission power supply” as greenhouse effect gas is not discharged during power generation, and is being planned to be introduced in large amount in the future throughout the world.
For instance, patent document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2007-274841) discloses a photovoltaic system and a photovoltaic plant comprising a solar battery array including a plurality of solar batteries, an inverter that converts the DC (direct current) power output from the solar battery array to the AC (alternating current) power, and a step-up transformer that steps up the AC power output from the inverter to high voltage. In the photovoltaic system having such configuration, the output is desired to be smoothened since the output fluctuates due to change in weather.
Patent document 2 (Japanese Patent No. 4170565) discloses a dispersed power supply system including: a power fluctuation smoothening device with a power storing unit, a charging/discharging unit, and a control unit; a dispersed power supply such as a photovoltaic solar cell; and a power converting device of a system interconnected type. In the dispersed power supply system having such configuration, the power storing unit is discharged when the output of the dispersed power supply is reduced and the power storing unit is charged when the output of the dispersed power supply is increased to thereby smooth the output.
In the synchronous power generator such as a fuel generator and a hydraulic generator that supplies power to the power system, the power generator characteristics of the power generator are fixedly defined once designed and manufactured. However, the conventional photovoltaic system and the power supply system that supplies the DC power and converts to the AC for output do not have the power generator characteristics of the synchronous power generator. The reasons thereof lie in that the conventional photovoltaic system and the like do not include a rotating portion such as the synchronous power generator, do not include a device for speed governing, and do not include a device of a so-called excitation system such as a voltage adjustment device.
Therefore, the conventional photovoltaic system and the power supply system cannot contribute to stable operation, that is, maintaining of stability, maintaining of frequency, and maintaining of voltage of the power system in cooperation with the synchronous power generator already introduced.